1. Field of the Invention
The present invention relates to electrofishing and, more particularly, to electric fish barriers for controlling movement of fish.
2. Description of Related Art
Electrofishing apparatus, to attract fish to a specific location or to repel fish from a specific location, have been used for decades. Electric barriers to control movement of fish have also been used for decades with greater or lesser degree of success. These barriers fall into two broad groups: those that use alternating current (AC) and those that use direct current (DC). In each of these two broad groups, the electric barriers may incorporate electrodes that lie flat on the bottom of the body of water or they may incorporate various patterns of immersed vertical rods or cables serving as electrode arrays. Various other configurations of electrodes are also known. A plethora of varieties of types of electric power sources exist, including the use of pulsed and constant currents (whether AC or DC), various frequencies or frequency ranges, single phase or three phase current, spacing and separation of electrode elements as a function of the type and nature of electric power applied, etc.
Each of the existing electrofishing apparatus and electric barriers attempts to provide a balanced solution of the various problems set forth below. Alternating current (AC) is used as the source of electric power for the following primary advantages: (1) permits more economy of design; (2) fish are far more sensitive to an AC field than to a DC field; (3) continuous use of DC is limited because of the electrolytic build up of oxides and other chemicals that occur on the cathode electrode resulting in electrically non conducting blankets enveloping the cathode electrode and there occurs erosion of the anode electrode; (4) it is easier to design uniformly distributed fields to eliminate equipotential paths through the electric field. Direct current (DC), whether pulsed or constant, offers primarily only one advantage. Fish swimming in a unipolar electric field of sufficient strength will suffer a type of seizure (galvanotropism) which causes the fish to swim only toward the anode electrode. This means that if the cathode electrode is identified as a "barrier", fish swimming toward the cathode electrode will turn and swim away (galvanotaxis); thus, the cathode electrode presents a type of barrier to fish movement. Sophisticated versions of DC electric fields involve use of a pulsed current, shaping of the pulses, selection of pulse frequencies and control of the width of the pulses. Pulsed DC current is more efficient than constant DC current in (1) causing neurological reactions in the fish; (2) reducing the amount of electrical energy consumed by the barrier; and (3) turning the fish away before they enter the strongest part of the field near the cathode electrode. If the barrier (cathode electrode) is in a slow moving or stagnant water, the galvanotaxis syndrome will cause the fish to move out of the strong electric field before they become narcotized (the next level of seizure that a fish experiences if shocked above the level that produces galvanotropism) and an accumulation of unconscious fish in proximity to the cathode electrode is prevented. In practice however, fish that are not moved out of the strong electric field by the moving water, swimming or electrotaxis, suffocate and die. The accumulation of dead fish quickly translates into a very unpleasant and unhealthy problem.
DC systems that are sufficiently sophisticated to accomplish the desired control of the movement of fish are expensive and subject to failure. Where the system is a fish barrier, it must be on continuously with zero down time or else it is ineffective in serving its primary purpose. Finally, the electrolytic action on the cathode and anode electrodes will ultimately cause down time for replacement unless significant redundancy of electrodes and circuits is employed.
The electrolytic action on the electrodes due to DC current can be avoided and the neurological reactions of the fish to DC current can be preserved if the electric field is formed by an asymmetrical AC current. In such systems, the total amount of energy in the positive and negative portions of each cycle is equal. By appropriate circuit design, it is possible to produce a brief high voltage pulse in the positive half cycle and a much lower peak voltage but longer duration pulse in the negative half cycle, whereby the energy of each half cycle is essentially equal. Such equality of energy in each cycle will effectively neutralize electrolytic build up upon the electrodes. Since the fish will respond neurologically to the peak voltage half cycles, the reactions of galvanotropism and subsequent galvanotaxis will be preserved. Accordingly, such a system is probably ideal for a fish barrier but it is technologically different to construct, is expensive and suffers from problems of reliability. Accordingly, it has limited practical applications.
Electrodes are normally set in a vertical array since such an array will produce a uniform distribution of an electric field which field assures that fish travelling at any depth in the adjacent water column will encounter a strong field strength. Such a configuration is unpopular and not very suitable for sites such as rivers that have moving water that may carry debris since such debris may collect at and build up upon the electrodes. With such build up, the force of the moving water may be sufficient to damage or destroy the electrodes. If the electrode spacing is sufficient to allow expected debris to pass, the effective electrical field between the electrodes is severely degenerated. To place electrodes horizontally on the bottom of the body of water or suspended in a water column, or both for deeper waters, the debris problem may be solved but the electric field produced is not uniform from the top to the bottom of the water. Without a uniform field, accurate prediction of fish reaction at any location in the electric field becomes problematical. That is, the fish travelling along the bottom will not be subjected to the same field strength as the fish moving close to the surface or anywhere therebetween. When the electrodes are horizontal, such as only on the bottom of the body of water, the voltage gradient is essentially vertically oriented and equipotential lines within the field are, to a great extent, horizontal. Accordingly, it is possible for a fish moving horizontally to sense no voltage difference between the voltage present at its head and at its tail. Without such a sensed voltage difference, the electric field will have little effect upon the fish. While there is no voltage gradient between the head and tail when the fish is parallel to the equipotential lines, there is a voltage from one side of the body of the fish to the other. This voltage gradient is small when compared to the fish with its entire body length across the field and the amount of current induced in its flesh is proportionately less. Still, in the strongest part of the field, a fish can receive enough of a shock in this position that all of the neurological reactions of galvanotaxis narcosis, and tetany can occur.
In many parts of the United States and the world, there exist interconnected bodies of water wherein an upstream body of water has been stocked with fish for sport fishing purposes or for fish farming purposes. When fish predatory to the stocked fish exist in a body of water downstream, it is mandatory to prevent movement of the predatory fish into the upstream body of water containing the stocked fish. In most instances, the use of a physical barrier to absolutely prevent such migration of the predatory fish is not a viable solution. Accordingly, a barrier which accommodates flow from the upstream body of water to the downstream body of water must be used. Moreover, such barrier must accommodate the flow of normal debris and detritus. To be effective, as discussed above, the barrier must operate continuously and for a period of years with minimal maintenance and replacement requirements. Finally, the barrier must accommodate very slow moving water and rapidly moving water without change in effectiveness.